Check if ctx/string really works with msvc
Fetch unit tests on github from https://github.com/lefticus/constexpr_all_the_things to do so. Check if hana::string can be a solution too.
This is needed to fix the concept_map implementation on https://gitlab.lrde.epita.fr/mroynard/metaprog/blob/dev/include/metaprog/cpt/verif.hpp#L39